1. Field of the Invention
The present invention relates to a sealed bearing roller cutter-type rotary drill bit used to drill oil and gas wells in the earth, and more particularly to such a bit having a bearing failure indicator feature.
2. Description of the Prior Art
Rotary drill bits of the roller cutter type, for example as disclosed in U.S. Pat. No. 3,923,348, (incorporated herein by reference) are the most commonly used type of drill bits in the oil and gas well drilling industry because they offer satisfactory rates of penetration and useful lives in drilling most commonly encountered formations. Roller cutter drill bits include a bit body having a threaded pin at its upper end adapted to be detachably secured to a drill string suspended from a drill rig, and a plurality of depending legs, typically three such legs, at its lower end. The drill bit further includes a plurality of conical roller cutters having cutting elements thereon, one for each leg, and a bearing rotatably mounting each roller cutter on its respective leg.
Sealed bearing type roller cutter bits further have a lubrication system including a reservoir holding a supply of lubricant. A passage in the bit body extends from the reservoir to the bearing to allow flow of lubricant to the bearing. A seal is disposed between the roller cutter and the bearing journal to hold lubricant in the bit. A diaphragm at the reservoir provides pressure compensation between the lubricant and the drilling fluid in the annulus between the bit and the well bore.
In use, roller cutter drill bits are rotated in the well bore on the end of a drill string which applies a relatively high downward force thereto. As the bits are rotated, the conical roller cutters rotate on the bearing journals, thereby bringing the cutting elements into engagement with the rock at the bottom of the well bore. The cutting elements drill the rock at the well bore bottom by applying high point loads to the rock, thereby causing it to crack or fracture in compression.
For most cost effective drilling, a worn drill bit should be replaced when the increased cost due to the worn bit""s reduced rate of drilling penetration, as compared to that of a new bit, becomes equal to the cost of replacing the bit (i.e., the cost of the new bit plus the cost of rig time in tripping the drill string in and out of the well bore). Unfortunately, once a drill bit is positioned in a well bore, it becomes hard to gather reliable information regarding its operating condition, its performance and its remaining useful life. Typically, the decision by a rig operator to replace a drill bit is a subjective one, based on experience and offset data showing the performance of similar bits in drilling similar formations. However, because of the many factors affecting drilling performance, besides the condition and performance of the bit itself, the rig operator""s decision as to when to replace a bit may not be correct.
The rig operator may unknowingly run the bit so long that it fails. Bit failure may also result from an improper application of the bit, such as by excessive weight on the bit, excessive rotational speed and drilling in the wrong kind of bit for the type of rock being drilled, or even from a defect in the bit itself. Bit failures typically occur in one of two modes: (1) breakage of the cutting elements, or (2) bearing failure. While the first mode is more common, the second may be more serious. In the first mode, pieces of the cutting elements, which are either steel teeth or tungsten carbide inserts, are broken from the roller cutters. This breakage significantly reduces the rate of drilling penetration, but the broken pieces are typically carried away from the well bore bottom by the circulating drilling fluid thereby leaving the well bore bottom clean for a replacement bit. In contrast, if the bit is continued to be used with a failed bearing assembly, the assembly will no longer be able to hold the roller cutter on the bearing journal and the roller cutter will fall from the bit when the drill string is pulled from the well bore. A lost roller cutter can be retrieved from the well bore bottom only by a time-consuming and expensive xe2x80x9cfishingxe2x80x9d operation, in which a special retrieval tool is tripped in and out of the well bore. In sealed bearing roller cutter bits, bearing failure is often the result of a seal failure which allows lubricant to flow out of the bit and drilling fluid, having abrasive particles entrained therein, to flow in. Although less common, diaphragm failure has the same result as seal failure. In any event, bearing failure is almost always preceded by or at least accompanied by a loss of lubricant.
Numerous bearing failure indicator systems have been proposed for inclusion in drill bits so as to give the rig operator a signal indicating bearing failure. One such system involves measurement and interpretation of certain drilling parameters at the drill rig, such as drill string torque, weight on bit, and rate of penetration, to signal drill bit bearing failure. In practice this system has proved to be unreliable.
From the foregoing it can be seen that there is a need for an apparatus that reliably detects bearing failure in roller cutter type rotary drill bits.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a rotary drill bit for attachment to a drill string defining a drilling fluid chamber. The rotary drill bit includes a bit body, at least one roller cutter, at least one duct and at least one plug. The bit body includes a top collar that facilitates attachment of the bit body to a drill string and a plurality of upright legs depending downwardly from the collar. Each of the plurality of legs is connected to a lateral wall, having an upper surface and an opposite lower surface, that separates the drilling fluid chamber from a lower open region defined by the lower surface. At least one of the plurality of legs terminates in an exterior bearing surface. The roller cutter is affixed to a corresponding exterior bearing surface of at least one of the plurality of legs and has an exterior cutting surface and a base edge. The roller cutter defines an interior bearing surface that is complementary to a corresponding exterior bearing surface of the leg and that opens to the base edge. At least one duct is defined by lower surface of the lateral wall and is adjacent to at least one exterior bearing surface. A plug, at least a portion of which is disposed in a portion of the duct, is disposed from the base edge one of the roller cutters at a distance so that if either the interior bearing surface or the exterior bearing surface wears beyond a predetermined threshold, the plug will cause a remotely-sensible indication of excessive bearing wear to be asserted.
In another aspect, the invention is a rotary drill bit for attachment to a drill string that defines a drilling fluid chamber. The rotary drill bit includes a bit body, a plurality of roller cutters each affixed to the bit body by a bearing, a plurality of ducts that communicate between the drilling fluid chamber and a lower open region and a plurality of brittle plugs that seal the ducts unless a bearing has worn beyond a threshold. The bit body includes a top collar that facilitates attachment of the bit body to a drill string and a plurality of upright legs depending downwardly from the collar. Each of the plurality of legs is connected to a lateral wall, having an upper surface and an opposite lower surface, that separates the drilling fluid chamber from a lower open region defined by the lower surface. Each of the plurality of legs terminates in an exterior bearing surface. The roller cutters are each affixed to a corresponding exterior bearing surface of a different one of the plurality of legs. Each roller cutter has an exterior cutting surface and a base edge, and defines an interior bearing surface that is complementary to a corresponding exterior bearing surface of at least one of the legs and that opens to the base edge. Each of the ducts is defined by the bit body and each duct opens to the upper surface of the lateral wall and to the lower open region adjacent each one of the legs. Each duct includes a first bore extending downwardly from the upper surface of the lateral wall and opening to the drilling fluid chamber. A second bore, having an open end, extends inwardly from the lower surface of the lateral wall from the open end and opens to the lower open region. The second bore intersects the first bore so that the lower open region is in fluid communication with the drilling fluid chamber.
The brittle plugs are typically hollow tungsten carbide shale burn inserts that seal each of the ducts from the lower open region adjacent the open end of the second bore. Each brittle plug is disposed adjacent the base edge of a different one of the roller cutters at a distance from the base edge so that if either the interior bearing surface or the exterior bearing surface wears beyond a predetermined threshold, the base edge will cause the brittle plug to allow drilling fluid to pass therethrough, thereby causing a drop in drilling fluid pressure in the drilling fluid chamber.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.